In a singly-fed induction machine the currents in the secondary winding (usually the rotor) are created solely by induction. These currents result from voltages induced in the secondary windings by rotating magnetic fields in the primary winding which arise from the application of line voltages thereto. These fields rotate at a speed, called synchronous speed, which is determined by the frequency of the applied voltage and the number of poles.
When the machine is operated as a motor, the rotor rotates at speeds below the synchronous speed. The difference in speed between synchronous and rotor speed is referred to as the slip speed, usually expressed as a per unit or percentage of the synchronous speed.
The rotor windings are generally connected to slip rings to which external adjustable resistances can be connected in series with the windings. These resistances limit the secondary currents during "start" of operation as a motor. As the motor picks up speed, the external resistance is gradually reduced whereby the efficiency increases. The resistance can also be used to control the speed; however, this method of speed control is very inefficient.
When the rotor is driven at above the synchronous speed, the machine acts as a generator. With resistance in the secondary windings, the output power can be maintained somewhat constant over a narrow range of rotor speeds.
Conventionally, when driving a generator with various types of prime movers, the speed of the electrical generator was kept nearly constant. Various mechanical methods for controlling speed have been used depending on the prime mover. When using an alternator with DC excited fields, for 60 Hz output frequency, the speed must be kept constant to a very close tolerance; i.e. to within one revolution per minute for synchronous speed of 1800 or 3600 revolutions per minute. When using a squirrel-cage induction generator, the most common generator for wind turbines, operation at a small percent above synchronous speed is necessary. If, inadvertently, a higher torque is supplied by the prime mover, the generator completely releases its load and a "runaway condition" exists. Under such circumstances, the prime mover, a wind or steam turbine, or a diesel, may race to destruction in a very few seconds.
However, variable speed generators are desirable for wind turbines to follow the changes in wind velocity and to reduce wear in the gear box which matches generator speed to turbine speed. Variable speed is also needed to eliminate voltage flicker caused by power surges from wind gusts.
For variable speed wind generators, power electronics may be used to control rotor voltage and frequency using the wound rotor induction machine. Power electronic controls tend to be complex with many components, they require feed-back circuits to match injected rotor frequency to turbine speed, and they may produce troublesome electrical harmonics. For wave generators, fixed resistors are being used to provide variable speed, with very poor efficiency.
In U.S. Pat. No. 2,648,808 there is described a motor having a wound primary winding (stator) in which the effective impedance of the primary windings is varied to improve the torque-speed characteristics of the motor. More particularly, the power factor of the motor is improved by controlling the impedance of the primary windings by adding thereto external series impedances.
U.S. Pat. Nos. 5,525,894 and 5,587,643 use secondary resistors and secondary capacitors for variable-speed wound rotor generator control. In the first patent, they are used for load limiting and in the second patent, they are used for increased power output and improved efficiency.
In generators which include slip rings and brushes, the slip rings and brushes are a common source of increased maintenance costs. The carbon brushes function best with a current density of about 60 amperes per square inch and with a certain amount of humidity in the surrounding atmosphere. With a fixed current density of about this value and reasonable humidity, the transfer of current between slip ring and brush is efficient. Under these conditions, an interchange of electrons takes place between the surfaces of the metallic slip rings and the carbon brushes which develops a lubricating film. This film allows current transfer with a very low voltage drop, with no heating, and with little friction between the fixed brushes and the rotating rings.
However, if the current density becomes too low, or if the humidity becomes too low, the lubricating film does not develop. The voltage drop, heating and friction all increase at the brush-ring interface. The carbon brush now acts as an abrasive against the slip ring, wearing both the slip ring and the brush. Carbon and metallic dust is created which conducts electricity. When it finds its way into the generator winding, winding failures can result.
With wind and wave generators, the generator load varies, depending on the amount of wind or wave activity, so it is impossible to maintain a constant current density in the carbon brushes. The wind and wave generators are not readily accessible. Wind generators are usually mounted high on a tower, and wave generators are at sea. Accordingly, there are very high maintenance costs associated with checking on the condition of the brushes and rings, replacing worn brushes, resurfacing slip rings, cleaning the windings of carbon dust, and in repairing failures.